1. Field of the Invention
The invention relates to an arrangement of thermal conductivity detectors that requires a minimum of space and is thermally symmetrical.
2. Description of the Related Art
Thermal conductivity detectors are used to detect certain liquid or gaseous substances (fluids) based on the thermal conductivity typical of the substance, and are particularly employed in gas chromatography. To this end, after the substances to be detected have been chromatographically separated they are fed one after the other in a measuring channel past an electrically heated heating filament arranged therein where, depending on the thermal conductivity of the substance flowing past, more or less heat from the heating filament is dissipated onto the channel wall and the heating filament is correspondingly cooled to a greater or lesser extent. As a result of the heating filament cooling down, its electrical resistance changes, which change is detected. To this end, the heating filament is normally arranged in a measuring bridge that contains further resistances and a further heating filament in a reference channel through which a reference fluid flows. Instead of the resistances, further heating filaments can be provided that are arranged fluidically parallel or in series with the heating filaments in the measuring channel or reference channel.
In the following, the term thermal conductivity detector is in each case used for a heating filament and the channel section surrounding the heating filament.
In order to maintain the thermal conductivity detectors used in a measuring bridge at the same temperature level, it is known, for example, from U.S. Pat. No. 2,512,857, U.S. Pat. No. 3,474,660 or DE 103 18 450 B3 to form the channel sections radially symmetrically around the heating filaments in a detector block that consists of a metal having a high thermal conductivity. The channel sections are furthermore arranged radially symmetrically around a central axis of the detector block to obtain a thermal symmetry.
As is known, for example, from WO 2009/095494 A1, thermal conductivity detectors can also be formed in a micromechanically produced component that can then be arranged in a recess in the detector block.
Depending on the application and complexity of the separating column switching or in the case of a plurality of analysis trains, a differing number of thermal conductivity detectors may be required in a gas chromatograph, where the detectors have hitherto been incorporated individually or as detector modules each having a plurality of thermal conductivity detectors in a detector block at different locations of the gas chromatograph.